Two very long copper bars with a square section $(6$ mm on each side$)$ must be joined with $56\%$ silver alloy solder $($melting temperature Tf $=618$C$),$ which offers a greater mechanical resistance and allows operation at higher temperatures. In both bars, h $=7$ W$/$m$2$K and T$\backslash$infty $=21$C$.$ Calculate the minimum power required in W to maintain the face contact between both bars at Tf$.$